clocksource: Make negative motion detection more robust

Guenter reported boot stalls on a emulated ARM 32-bit platform, which has a 24-bit wide clocksource. It turns out that the calculated maximal idle time, which limits idle sleeps to prevent clocksource wrap arounds, is close to the point where the negative motion detection triggers. max_idle_ns: 597268854 ns negative motion tripping point: 671088640 ns If the idle wakeup is delayed beyond that point, the clocksource advances far enough to trigger the negative motion detection. This prevents the clock to advance and in the worst case the system stalls completely if the consecutive sleeps based on the stale clock are delayed as well. Cure this by calculating a more robust cut-off value for negative motion, which covers 87.5% of the actual clocksource counter width. Compare the delta against this value to catch negative motion. This is specifically for clock sources with a small counter width as their wrap around time is close to the half counter width. For clock sources with wide counters this is not a problem because the maximum idle time is far from the half counter width due to the math overflow protection constraints. For the case at hand this results in a tripping point of 1174405120ns. Note, that this cannot prevent issues when the delay exceeds the 87.5% margin, but that's not different from the previous unchecked version which allowed arbitrary time jumps. Systems with small counter width are prone to invalid results, but this problem is unlikely to be seen on real hardware. If such a system completely stalls for more than half a second, then there are other more urgent problems than the counter wrapping around.

TGThomas Gleixner committed 1 year ago